Maintaining Grid Reliability with a High Renewables Portfolio

10/01/2012 | Joe Zwers

Many governments have adopted the goal of achieving 20% renewable power generation by 2020. For California, the goal is to reach 33% by 2020.

The state definitely has the sunshine needed, and in addition to 2,000 MW of photovoltaic installations, there are the 354 MW of Solar Energy Generating Systems that have been operating in the Mojave Desert since the 1980s, soon to be joined by the 392-MW Ivanpah concentrating solar thermal plant currently under construction.

The state also was the first to install utility-scale wind generation. Although Texas and Iowa have since passed California in terms of installed capacity, California added 921 MW in 2011, bringing its total to 3,927 MW. According to the American Wind Energy Association, another 18,269 MW of wind projects are in the queue.

But alternative energy sources alone are not enough to provide reliable generation or to stabilize the grid.

“One of the main issues that we are trying to raise awareness about is the need for flexible capacity on the system,” said Stephanie McCorkle, spokesperson for the California Independent System Operator (CAISO). “Gas-fired generation is our only means right now until storage and demand response has matured to compensate for the fluctuations of wind and solar power, so we are actively educating policy makers and trying to propose a solution at the regulatory level as well to ensure that there is still enough conventional gas-fired generation available to us to be able to maintain reliability as we see more and more renewables added to the grid.”

This includes having enough reactive power to stabilize the grid and allow the remotely generated renewable energy to reach population centers. Both the generation and transmission issues are easily addressable with existing technology.

A Problem of Predictability

4. Balancing act. This is the expected typical April load profile for CAISO in 2020, the year when California’s 33% renewable portfolio standard becomes effective. When the sun shines during the day, and the wind blows in the afternoon, conventional thermal resources have to be ramped down. But when demand spikes in the morning and early evening, conventional resources have to be dispatched (the red line) to supply the needed power. Source: CAISO

The first problem with high renewable penetration is that wind and solar are not dispatchable. Figure 4 from CAISO shows a projected mix of fossil, wind, and solar energy generation smoothly ramping up and down throughout the day to meet changes in load. If that worked in the real world, there would be no problem. Figure 5, however, shows how it works in the real world. Although wind power may average out to a nice smooth curve, on an hourly and daily basis, it is quite variable.

Solar is similarly unpredictable. On a day-to-day basis, you may know that it will be cloudy or sunny, but each passing cloud drops the output on a photovoltaic array. “It is difficult to predict what wind is going to do from one day to the next,” said Clyde Loutan, senior advisor for the market and infrastructure development at CAISO. “The big challenge is balancing wind and solar in real time due to sub-hourly variations.”

He said that CAISO can lose 500 MW in 5 minutes due to wind drop or cloud cover. To improve the ability to rapidly respond to these changes, CAISO has set up a new control center with a dedicated renewables dispatch desk, which is manned 24/7 in an attempt to stay ahead of changes in wind and solar output.

“We have a lot of high-tech tools in the data center such as geospatial map tech, so we can see changes in weather conditions, wind resource areas, and solar resource areas and see output in real time,” said McCorkle. “The visuals will show changes in how much of installed capacity is actually producing so our renewable dispatchers can try to put a megawatt number on either the increase or decrease and we can fire up the gas-fired generators to compensate for that.”

To make this work requires a full complement of fast-start gas turbines throughout the state ready to come online at a moment’s notice. This is why roughly half of the LMS100s GE has sold to date have been installed in California (including, for example, the Panoche Energy Center in Firebaugh, Calif., which is profiled in the September 2011 issue, available at http://www.powermag.com).

“The nature of this fluctuating renewable power requires other generation to respond quickly to match the total load demand and maintain grid stability,” said GE LMS100 Product Manager Phil Tinne. “The LMS100 machine is ideally suited to this task: It can ramp power at 50 MW/min over a very wide load range while staying in emissions compliance [with exhaust system treatment for California’s strict standards].”

Renewables and Transmission

In addition to variability, the other problem posed by renewables is that these generation sources are often sited far away from the load, increasing the need to provide more reactive power.

“When you go out into the desert and mountains to produce solar and wind energy, this power has to travel great distances on the transmission lines, which in many cases don’t even exist yet,” said Ken Buttke, Jr. a power generation manufacturers’ representative with South Port Equipment in Agoura Hills, Calif. “As you load up these lines, you build up resistance, but by using synchronous condensing, you can boost the power factor up to a higher level and stabilize the voltage.”

A simple way to achieve this is to use the quick-start gas turbines as synchronous condensers. By putting a clutch between the turbine and the generator, the generator can stay synchronized to the grid, providing the necessary reactive power. Then, when active power is needed to compensate for a drop in renewable output, the turbine can quickly fire up, the clutch engages, and the unit starts generating electricity. Because the generator is already synchronized to the grid, the unloaded turbine can reach synchronous speed quicker, and there is less wear on the turbine from the frequent stops and starts. (See “How to Make VARs—and a Buck,” June 2007 and “CFE Extends CTG Universidad Unit 2’s Life with Conversion to Synchronous Condenser,” August 2011 in the POWER archives for more information on the value of synchronous condensers.)

Utilities throughout the world have been installing clutches that allow idle generators to provide synchronous condensing for more than 30 years. Here are four examples of how this approach has worked to address different situations:

British Columbia. Thirty-two hydroelectric plants provide BC Hydro with 87% of its generating capacity. To correct the poor power factor of the electricity coming from the remote generators, BC Hydro converted the six 150-MW steam turbine generators at a thermal plant in the Vancouver area to operate as synchronous condensers. From April to October, these generators provide electricity, and water is allowed to build up behind the dams. During the other half of the year, when natural gas is needed for residential heating, five of the generators are disconnected from the turbines and provide voltage support for the hydro power. Not only did this improve stability in Vancouver, it also improved the power factor enough to allow the utility to sell more power to U.S. customers.

Western Australia. In Western Australia, three-quarters of the population lives in or near the city of Perth, but much of the generation has switched to steam plants located at coal mines, cogeneration facilities, alumina refineries, and the 180-MW Emu Downs Wind Farm, all of which are located about 200 kilometers from the city center. To provide the necessary reactive power near Perth, Verve Energy recently retrofitted three of the nine GE Frame 6 turbines at its Pinjar Gas Turbine Power Station with clutches so they can operate as synchronous condensers when not generating power, joining the three units that originally came with clutches 20 years ago.

Los Angeles. The Los Angeles Department of Water and Power (LADWP) is in the middle of a repowering project at its Haynes Power Station. Although the project was mandated by a state regulation limiting the use of seawater for cooling, LADWP is using switching to GE LMS100 generators to give it the quick-start capabilities needed to support its increasing renewables portfolio and is installing clutches on two of the units so they can provide necessary voltage support when peaking isn’t required.

Key West. The City of Key West, Fla., has its own local generating station but found it was more economical to import power from the mainland. In 1987 it started using dual 138 kV transmission lines to connect with a Florida Power & Light station 185 miles away. However, because the City of Key West was at the end of a radial system and lacked voltage support, the transmission lines lacked the ability to carry enough power to meet rising demand. In 1997, Key Energy Services converted a decommissioned steam generating unit into a synchronous condenser, boosting the transmission lines’ capacity by 34 MW and improving stability.

Some ISOs, including PJM Interconnection and the Midwest ISO, have markets for synchronous condensing, which make it profitable for power producers to install clutches and provide voltage support. In California—although LADWP, which manages its own grid, is setting up some of its new LMS100s to provide synchronous condensing—for the state as a whole, CAISO does not have such a market, which limits the incentive for independent power providers to provide voltage support. However, as California looks to increase its renewable sources and back them up with gas turbines, such a market may be needed to provide the incentive for producing low-cost reactive power to help stabilize the grid.

— Contributed by Joe Zwers, a freelance writer from Glendale, Calif., specializing in engineering and technology.

Videos & Infographics

GE Continues to Improve Gas Turbine Efficiency

Guy DeLeonardo, leader of gas turbine products for GE’s Gas Power Systems, provided POWER with an exclusive explanation of how the new nozzle design for the improved 9HA.02 gas turbine works. GE Power in early December 2017 said its 9HA.02 gas turbine reached a new milestone by exceeding 64% efficiency in combined cycle power plants. The company attributes at least part of the achievement to advances in additive manufacturing (3-D printing).